Field of the Invention
This invention relates to a method of determining the planarity or flatness, of a moving rolled strip. More particularly to the case of a metal strip, or steel strip, leaving a rolling mill stand.
Description of Prior Art
A method is already known, inter alia from U.S. Pat. No. 4,541,723, in which a plurality of points or the moving strip surface are surveyed by optical triangulation, by means of laser beams, and indices of the planarity, or flatness, of the strip are determined. As a general rule, the points surveyed with this system lie along the median axis of the strip and two or four lateral axes parallel to the median axis, depending on the strip width.
In this known method, the sighting points can be displaced laterally so as to cover the width of the strip whenever the width of the strips for rolling changes. Normally, once the position of these sighting points has been established for a strip of given width, it remains unchanged during the rolling of this strip or a number of successive strips of the same nominal width.
However, the strip frequently undergoes transverse movements during rolling, with the result that its median axis moves away from the virtual line described on the strip by the fixed central sighting point. The same shift of course affects the other axes surveyed.
This leads to a serious disadvantage because the planarity indices determined from these incorrect measurements are no longer sufficiently representative of the real planarity of the strip.